(1) FIELD OF THE INVENTION
This invention relates to a process for improving the properties of polymer emulsions.
(2) Description of the Prior Art
Increasing social demands for the prevention of environmental pollution and the saving of resources has tended to restrict the use of organic solvents in recent years. Therefore, the availability of polymer emulsions using water as a medium has been noted in fields formerly using large amounts of organic solvents such as coatings, adhesives and so forth. Furthermore, the polymer emulsion has the significant characteristics that it can be easily synthesized and, in addition, easily handled because its viscosity is much lower than that of the polymer solution having the same concentration. Therefore, polymer emulsions such as acrylic resin emulsions, polyvinyl acetate emulsions, ethylene-polyvinyl acetate copolymer emulsions and the like are used in a wide variety of applications, for example not only as coatings and adhesives but also as tackifiers finishing and treating agents for paper, fibers, woven fabrics leather, etc., and also as bonding agents for non-woven materials.
However, films obtained by drying such emulsions cannot eliminate the important defect that such films easily absorb moisture and swell and consequently their physical properties and adhesion to substrates fall remarkably. And furthermore, as is clear from the film-forming process, the film is one in which the polymer particles are coalesced, and it is, therefore, inferior to solvent-cast film in physical properties, especially, strength and toughness. For these reasons, the conventional polymer emulsion was unsatisfactory for the applications mentioned above.
In order to eliminate the defects of the conventional polymer emulsion, a thermosetting acrylic emulsion has been proposed comprising copolymer containing ionic reactive acrylic monomer and crosslinking agent bringing about ionic addition reaction or condensation reaction. It is reported that this type of thermosetting polymer emulsion forms a tough film by reacting the functional group in the copolymer with an ionic crosslinking agent at the time of drying the thermosetting polymer emulsion on heating.
However, since the monomer having ionic reactive functional group is, in general, strongly hydrophilic, the ionic functional group is apt to be localized on the surface of the copolymer. And furthermore, the ionic crosslinking agent is added in the form of water solution or o/w emulsion, and therefore, the crosslinking reaction at the time of drying by heating is restricted to the surface of the copolymer particles, and accordingly, the degree of crosslinking of the interior of the copolymer particles is extremely low. Therefore, an excess of the crosslinking agent must be used, which causes the formation of rigid and brittle film.
A thermosetting polymer emulsion free from the various defects mentioned above has, therefore, long been desired by those skilled in the art.